Most officers end up pulling over an impaired driver at some point during their shift. As soon as they see signs of impairment, the routine questions start and the routine Standardized Field Sobriety Tests are performed, one of which is the HGN.
I see a lot of officers use a pen light, ink pen, clip-on handcuff key, etc. as a stimulus for the Horizontal Gaze Nystagmus test. Per the National Highway Transportation Safety Administration (NHTSA) the stimulus needs to be held approximately 12 to 15 inches away from the eyes--close enough for a suspect to quickly grab whatever you’re holding in your hand. A hard, sharp, pointed object can serve as an effective assault tool against you.
Try using your non-dominant hand and the tip of your thumb instead. You reduce the opportunity the suspect has to stick something in your face or throat. If you absolutely need a stimulus, try a three inch piece of plastic drinking straw. It will fit right over the ballpoint pen you keep in your lapel.